The present invention relates to fibrous nonwoven webs, especially those that comprise polyethylene terephthalate fibers.
Direct formation of polymeric material into fibrous nonwoven webs by processes such as meltblowing has many advantages; but the strength properties of meltblown fibers can be less than desired. The polymer chains in meltblown fibers are generally not oriented sufficiently to provide a high level of strength properties to the fibers; see Encyclopedia of Polymer Science and Engineering, John Wiley and Sons, Inc., 1987, Volume 10, page 240. Meltblown fibers are typically prepared by extruding molten polymer through orifices in a die into a stream of high-velocity air which rapidly and greatly attenuates the extrudate to form generally small-diameter fibers. Much of the extension of the extrudate occurs while the polymer is above its melt temperature (Tm), with the result that the polymer molecules can relax some of the internal stresses generated during attenuation of the extrudate, and hence, may not achieve the rather high degree of orientation that can induce the molecules to form an ordered crystalline state.
Meltblown polyethylene terephthalate (PET) fibers are especially subject to the above tendencies. Collected PET meltblown fibers exhibit almost a total lack of crystalline orientation, because PET has a relatively high rate of relaxation, a relatively low rate of crystallization, a relatively high melt temperature, and a glass transition temperature (Tg) well above room temperature.
The lack of crystalline order weakens conventional meltblown PET fibers, and it also makes the fibers dimensionally unstable when exposed to elevated temperatures above their Tg. Some internal stressesxe2x80x94sometimes termed amorphous orientation, i.e., an orientation insufficient to induce crystalline orderxe2x80x94are produced during attenuation of the meltblown extrudate and are frozen in due to rapid quenching of the melt. Later heating of a nonwoven web of the fibers can release the internal stresses and allow the polymer chains to contract, whereupon the fibers shrink. Shrinkage at elevated temperatures can approach 50% of the web""s as-collected dimensions. In addition to contraction of the PET molecules upon exposure to elevated temperature, some crystallization of the molecules occurs; but this crystallization of the generally amorphous molecules actually embrittles and weakens the fibers.
The result is that while PET has a number of important advantagesxe2x80x94for example, it does not melt or degrade when exposed to rather high temperatures such as 180 degrees C., has desired flame retardancy as compared with polyolefins, and is of relatively low costxe2x80x94its use as a meltblown fiber has been limited.
Several attempts have been made to provide a more stable and useful meltblown PET fiber. U.S. Pat. No. 5,958,322 teaches a method for giving an already collected meltblown PET web increased dimensional stability by annealing the web while it is held on a tentering structure. While good dimensional stability is achieved by this technique, the process requires an extra processing step that adds expense; and greater improvement in morphology and strength would be desirable. Japanese Kokai No. 3-45768 is another teaching of heating a PET web or fabric under tension to increase crystallinity, with similar deficiencies.
U.S. Pat. No. 4,988,560 teaches a technique for orienting meltblown fibers, and achieves high-strength fibers. But the fibers described in that patent require special steps to gather and hold them into a coherent web, such as embossing the assembled fibers or adding a binder material to the assembled fibers. U.S. Pat. No. 4,622,259 similarly discusses high-strength meltblown fibers that require embossing or the like to consolidate assembled fibers into a handleable and usable web.
Japanese Kokai 90663/1980 (as described in European Patent No. 527,489, page 2, lines 36-51) teaches preparation of PET fibrous webs by a meltblown process which, in combination, uses high-pressure air blown through a narrow gap, PET polymers having an intrinsic viscosity of 0.55 or higher, and extrusion at a melt-viscosity higher than xe2x80x9cassures good melt-blowing condition.xe2x80x9d The process is said to provide PET meltblown fabric of good properties, such as strength, hand and thermal resistance; but EP 527,489 states that the process is commercially impractical and non-uniform, and that the fibers prepared lack adhesion with one another, and instead scatter during collection.
EP 527 489 itself seeks to overcome the deficiencies of the prior art by blending polyolefin into the PET polymer in an amount of 2-25 weight-percent. The polyolefin is said to become dispersed into the PET as discrete islands, resulting in a reduction in melt-viscosity, which, together with use of low-pressure air, is said to produce dimensionally stable meltblown fabrics.
U.S. Pat. No. 5,753,736 takes a different approach, using certain nucleating agents in PET to prepare meltblown PET webs having a combination of crystalline, amorphous and rigid amorphous molecular portions said to achieve shrink-resistance.
None of the above techniques is known to have resulted in actual, commercial, dimensionally stable meltblown fibrous PET webs. Despite significant prior effort, the need for such webs continues to be unsatisfied.
The present invention provides new nonwoven fibrous webs having excellent strength, durability and dimensional stability in comparison to conventional nonwoven webs. The fibers in these new webs are preferably meltblown PET fibers, and are characterized by a morphology that appears unique in such fibers. Specifically, the new fibers of the invention exhibit a chain-extended crystalline molecular portion (sometimes referred to as a strain-induced crystalline (SIC) portion), a non-chain-extended (NCE) crystalline molecular portion, and an amorphous portion. While not being bound to theoretical explanations, it is believed that the chain-extended crystalline portion in the new meltblown PET fibers of the invention provides unique, desirable physical properties such as strength and dimensional stability; and the amorphous portion in these new fibers provides fiber-to-fiber bonding: an assembly of the new fibers collected at the end of the meltblowing process may be coherent and handleable, and it can be simply passed through an oven to achieve further adhesion or bonding of fibers at points of fiber intersection, thereby forming a strong coherent and handleable web.
The unique morphology of the meltblown PET fibers of the invention can be detected in unique characteristics, such as those revealed by differential scanning calorimetry (DSC). A DSC plot for PET fibers of the invention shows the presence of molecular portions of different melting point, manifested as two melting-point peaks on the DSC plot (xe2x80x9cpeakxe2x80x9d means that portion of a heating curve that is attributable to a single process, e.g., melting of a specific molecular portion of the fiber such as the chain-extended portion; DSC plots of PET fibers of the invention show two peaks, though the peaks may be sufficiently close to one another that one peak is manifested as a shoulder on one of the curve portions that define the other peak). One peak is understood to be for the non-chain-extended portion (NCE), or less-ordered, molecular fraction, and the other peak is understood to be for the chain-extended, or SIC, molecular fraction. The latter peak occurs at a higher temperature than the first peak, which is indicative of the higher melting temperature of the chain-extended, or SIC, fraction. We are not aware of any previous nonwoven web comprising PET fibers that exhibit dual melting peaks on a DSC plot as described, and such webs offer superior propertiesxe2x80x94e.g., combined dimensional stability and toughnessxe2x80x94as will be further explained herein.
An amorphous molecular portion generally remains part of the PET fiber, and can provide autogenous bonding (bonding without aid of added binder material or embossing pressure) of fibers at points of fiber intersection. This does not mean bonding at all points of fiber intersection; the term bonding herein means sufficient bonding (i.e., adhesion between fibers usually involving some coalescence of polymeric material between contacting fibers but not necessarily a significant flowing of material) to form a web that coheres and can be lifted from a carrier web as a self-sustaining mass. The degree of bonding depends on the particular conditions of the process, such as distance from die to collector, processing temperature of molten polymer, temperature of attenuating air, etc. Further bonding beyond what may be achieved on the collector is often desired, and can be simply obtained by passing the collected web through an oven; calendering or embossing is not required but may be used to achieve particular effects.
In brief summary, a new PET-based web of the invention generally comprises a mass of PET fibers that a) exhibit dual melting peaks on a DSC plot representative of a first molecular portion within the fiber that is in a non-chain-extended (NCE) crystalline form, and a second molecular portion within the fiber that is in chain-extended crystalline form and has a melting point elevated over that of the NCE crystalline form, and b) are autogenously bondable.
For most uses of webs of the invention, the PET fibers preferably are of microfiber size, i.e., have an actual average diameter of 10 micrometers or less. However, larger fibers are satisfactory for many uses. Most often, the effective fiber diameter (EFD, measured by a technique that generally indicates a larger size than actual diameter) is 20 micrometers or less.
Also, for most uses, the web preferably has a density of less than 100 kilograms per cubic meter, though preferably more than 2 kg/m3. The pressure drop through the web is preferably at least 0.3 mm H2O pressure drop (as measured by passing a stream of air through a 102.6-square-centimeter area at a face velocity of 3.12 meters per minute), and more preferably at least 0.5 or 1 mm water. Such a pressure drop is characteristic of webs that exhibit good sound insulation properties. Sound insulation webs generally have a density of 50 kilograms per cubic meter or less, and preferably of 25 kilograms per cubic meter or less, and are preferably at least 1 or 2 centimeters thick.
Webs of the invention are generally used in an annealed form, which provides increased stability. In contrast to prior-art PET webs which have been annealed to achieve a degree of dimensional stability, but which become embrittled and weakened by random crystal growth during the annealing process, PET webs of the invention retain good strength and durability after the annealing process. Annealed webs of the invention also have enhanced bonding, and these bonds are retained well upon heat-exposure.
Webs that combine excellent dimensional stability and excellent strength, toughness and durability have been obtained. For example, webs with a shrinkage of no more than about 2% when exposed to a temperature of 160 degrees C. for 5 minutes have been obtained. In general, webs that shrink less than 20% under such conditions can be useful, though shrinkages of 5% or less are especially useful. Also the webs of the invention retain excellent strength, toughness and durability after annealing, even when measured after a time of storage, e.g., one month at ambient conditions. Webs of the invention comprising polyethylene terephthalate fibers offer high strength, good modulus (e.g., stiffness) and good loft properties, low release of volatile organic compounds upon heating, maintenance of physical properties upon thermal and environmental exposure, relatively low flammability, formability into micro-sized diameters, and lower cost. With the heat-resistance achieved by the present invention, meltblown PET webs of greatly increased utility are provided.
Webs of the invention are prepared by a new meltblowing method. The new method comprises the steps of extruding molten PET polymer through the orifices of a meltblowing die into a high-velocity gaseous stream that attenuates the extruded polymer into meltblown fibers, and collecting the prepared fibers, briefly characterized in that the extruded molten PET polymer has a processing temperature less than about 295 degrees C., and the high-velocity gaseous stream has a temperature less than the molten PET polymer and a velocity greater than about 100 meters per second. Preferably, the PET polymer has an intrinsic viscosity of about 0.60 or less.
In some methods of the invention, other fibers are dispersed among the PET fibers before they are collected. For example, crimped and/or uncrimped staple fibers may be dispersed among the meltblown PET fibers to achieve a more lofty or a more resilient web or to assist the web to be later molded and bonded in the molded shape (webs of the invention can usually be molded without presence of staple fibers).
While the invention is particularly applicable to polyethylene terephthalate, it is also useful with other semicrystalline polymeric materials, such as polyamides, polyolefins, and other polyesters. Processes of the invention better compensate against the effects of die swell (expansion of the extrudate as it leaves the die orifice, meaning that there is less relaxation of the polymer chains in the extrudate, and that lesser relaxation, together with the strain imposed as the extrudate solidifies in the high-velocity air, results in favorable crystalline properties for these polymers also.